(1) Field of the Invention
The present invention relates to a preparation process of cinnamic acids by the hydrolysis of a alkyl ester of the cinnamic acids.
Cinnamic acids are technically important as materials for perfumes, photosensitive polymers and various organic or biochemical products.
(2) Description of the Prior Art
Regarding the processes for the synthesis of cinnamic acid and its derivatives, a variety of methods has been known up to the present time.
For instance, these processes are methods of preparing cinnamic acid and its derivatives from benzaldehydes as a main raw material by Perkin reaction, Knoevenagel reaction or Claisen condensation (as described in, for example, Organic Reactions, vol. 1, 217 (1942)), and from benzene or its derivatives and acrylate esters as raw materials (as described in, for example, Japanese Patent Publication No. 50611/'72, Japanese Patent Laid-Open No. 59927/'83, U.S. Pat. No. 3,783,140 and U.S. Pat. No. 3,922,299). Besides a method has also been proposed recently wherein styrene or its derivatives are reacted with carbon monoxide, alcohol and oxygen in the presence of a catalyst (as described in Japanese Patent Publication Nos. 5570/'84 and 23661/'85).
Cinnamate esters are always formed as an intermediate in such processes as Claisen condensation which used benzaldehyde and acetate esters, a method of using benzene or its derivatives and acrylate esters as the raw materials, and a method of employing styrene or its derivatives, carbon monoxide, alcohol and oxygen as the raw materials. Therefore hydrolysis of cinnamate esters is necessary for preparing free cinnamic acids.
Concerning the hydrolysis of cinamate esters a method has recently been proposed which applies an acid as the catalyst (as described in Japanese Patent Laid-Open No. 112736/'85). The hydrolysis, however, is generally performed by use of alkali such as sodium hydroxide in a homogeneous system which contains aqueous mixture of alcohol, dioxane or acetone (as described in Japanese Patent Laid-Open No. 102614/'74).
On the hydrolysis of cinnamate esters with an acid catalyst a long reaction time is generally required because of a low reaction rate. On the other hand, the alkali hydrolysis is, as described above, carried out in a homogeneous system employing the aqueous mixture of organic solvents. Therefore, procedures such as extraction and concentration are required for the isolation of desired products. Furthermore, the solvents used for the extraction should be recovered in the industrial application.
Besides a step of acidifying the resultant alkaline solution with mineral acids after alakli hydrolysis is required in order to finally obtain the cinnamic acids.
In the prior arts, mineral acids are added to the aforesaid alkaline solution of alkali cinnamate containing the organic solvents.
In these cases, however, acidification must be performed in a low concentration which is diluted with a large amount of water as a result of employing a dilute aqueous alkali cinnamate solution, using a dilute aqueous solution of the acid or diluting the system with a special addition of water. Otherwise the system becomes a slurry having a high viscosity on the way of acidification and in extreme cases stirring is inhibited by the solidification of total system. Therefore it becomes difficult to conduct the acidification of aqueous solution of the alkali cinnamate with the mineral acid in a high concentration and the amount of products produced per unit volume of the reaction vessel is reduced.
Furthermore, the use of organic solvents described in the prior art results in the loss due to dissolving of desired product into the solvents and hence cause the reduction of yield.